Solar cells have been developed for generating electrical energy directly from sunlight. In general, these cells can be classified as either heterojunction devices, which depend upon junctions such as those formed between two different semiconductor materials or between a metal and a semiconductor or from a metal/insulator/semiconductor sandwich, and homojunction devices which depend only upon junctions formed between layers of the same semiconductor material doped with different impurities to provide different electrical properties.
Heretofore, homojunction cells using direct-gap semiconductor materials have generally exhibited disappointing efficiencies. One reason for the relatively low efficiencies in homojunction solar cells is believed to be the high absorption coefficient which is inherent in direct gap semiconductor materials such as gallium arsenide. For example, approximately half of the carriers due to AM 1 radiation are generated within 0.2 .mu.m of the surface of gallium arsenide. Therefore, for materials such as GaAs, which also has a high surface recombination velocity, most of the carriers generated by solar radiation recombine before they reach the junction, causing a significant decrease in conversion efficiency.
One approach which has been used to overcome this problem has been the use of a thin window layer of gallium aluminum arsenide (Ga.sub.l-x Al.sub.x As) grown over the GaAs wafer by liquid phase epitaxy. Such cells may be referred to as heteroface cells. Because the recombination velocity is much less at a Ga.sub.l-x Al.sub.x As/GaAs interface than at a GaAs surface, higher conversion efficiencies have been achieved. Thus, Hovel and Woodall report conversion efficiencies of up to 22% for Ga.sub.l-x Al.sub.x As/GaAs heteroface solar cells but only up to 14% for GaAs homojunction solar cells for air mass 1 (AM 1) radiation. See Hovel and Woodall, J. M., 12th IEEE Photovoltaic Specialists Conf., 1976 (Institute of Electrical and Electronic Engineers, New York, 1976), p. 945.
Nevertheless, aluminum is so reactive in the vapor phase that it is difficult to prepare high quality Ga.sub.l-x Al.sub.x As layers by conventional chemical vapor deposition, which is a highly preferred fabrication method. Because of this, it has been necessary to grow Ga.sub.l-x Al.sub.x As.sub.x layers by metal-organic chemical vapor deposition. See Dupuis, R. D., Dapkus, P. D., Yingling, R. D. and Moody, L. A., Appl. Phys. Lett., 31 201 (1977). This method can be both more expensive and more time consuming than conventional chemical vapor deposition.